Field of the Invention
The present invention relates to a bit expansion method and apparatus that do not cause a picture deterioration when displaying with expanding a number of bits.
Discussion of the Related Art
In picture displays such as a TV and a PC, R, G and B data each mostly have a 8-bit precision. When a certain picture signal processing is conducted for an input image (R, G and B data of which each have 8 bits) and then the image is displayed, a reliable bit length becomes less than a 8-bit length due to accumulation of operation error, and thus there occurs a false contour.
A noticeable example of a picture signal process is a gamma correction process. However, in the gamma correction process, there is reduction of a bit precision when converting input 8 bits with a LUT (look-up table) or the like.
To solve the problem, there is a method that an 8 bit input is expanded to a 10 bit then is inputted to a LUT. FIG. 8 is a block diagram of a picture signal process apparatus according to the related art. Concretely, it is shown that an input picture of 8 bits is expanded to a picture of 10 bits, then a gamma correction process is conducted, and then the processed picture is outputted to a display panel. By conducting a bit expansion process and then conducting a desired operation process such as a gamma correction or the like, suppressing a deterioration of a bit precision is considered.
However, the related art has problems in operation precision and grey level conversion as follows.
(1) As to a Problem in Operation Precision:
A multiplication is a main operation in a filter or the like. For the sake of brevity, in a fixed-length operation, a multiplication of an 8 bit input by an 8 bit coefficient is assumed. FIG. 9 is a view explaining a state that a deterioration of precision occurs by conducting a multiplication in the related art picture process apparatus. Even though a bit precision is maintained in intermediate operation, when a rounding process of 8 bits is finally conducted, a significant bit number becomes 7 bits.
Further, in case of using 2-stage filter, a significant bit number is further reduced by 1 bit and becomes 6 bits. As operation is repeated, a significant number is reduced, and thus a picture has a 6 bit or 5 bit precision. Accordingly, there occurs a problem that a false contour is seen at a gradation portion.
(2) As to a Problem in Grey Level Conversion:
For example, regarding a gamma correction, in a gamma correction of 8 bit input and 8 bit output, there is a case that an existence range of pixel value is discontinuous. FIG. 10 is a view explaining change of histogram when conducting a grey level conversion in the related art picture signal process apparatus. FIG. 10A shows a conversion curve when obtaining 8 bit output data by conducting a gamma correction for 8 bit input data. FIG. 10B shows a histogram of an input picture, and FIG. 10C shows a histogram of a picture after a gamma correction. As is plain based on comparison of FIG. 10B with FIG. 10C, a discontinuous change in frequency distribution of pixel value after a gamma correction happens, and thus there is a case that smoothness of an original picture is lost.
The summary of the above matters is as follows. For example, there is a case that a brightness adjustment and color tone adjustment, or grey level conversion or filter process for display quality improvement is conducted for an 8 bit input picture. However, in such the operation, a rounding error of operation is accumulated due to limitation of hardware, and thus there is a case that a reliable bit precision is less than 6 bits or 5 bits. As a result, it is problematic that a display quality deterioration such as a false contour or the like is seen at a gradation portion.
Further, this problem is not improved by simply bit-expanding an input picture, and, to prevent a picture deterioration, it has been required that a bit expansion process is conducted appropriately according to a brightness distribution state of an input picture.